1. Field of the Invention
The invention relates to a micromachined device, and more particularly, to a capacitor-based micromachined sensor.
2. Description of the Related Art
Microfabrication, also known as micromachining, commonly refers to the use of known semiconductor processing techniques to fabricate devices known as micro-electromechanical systems (MEMS) or micromachined devices. In general, known MEMS fabrication processes involve the sequential addition and removal of layers of material from a substrate layer through the use of film deposition and etching techniques until the desired structure has been realized. Accordingly, MEMS devices typically function under the same principles as their macroscale counterparts. MEMS devices, however, offer advantages in design, performance, and cost in comparison to their macroscale counterparts due to the decrease in scale of MEMS devices. In addition, due to batch fabrication techniques applicable to MEMS technology, significant reductions in per unit cost may be realized.
Micromachined structures are frequently used in MEMS inertial sensors, such as accelerometers and gyroscopes. A MEMS accelerometer using differential capacitors to detect acceleration typically includes three primary micromachined elements: a central, or proof mass, capacitor plates, and springs. FIG. 1 is a top plan view of a typical prior differential capacitor-based micromachined accelerometer 100, including a movable proof mass 102 supported by spring support beams 104. The proof mass 102 includes a plurality of electrodes 108 extending perpendicularly away from the proof mass 102, which are interleaved with a plurality of electrodes 110 extending perpendicularly from support beams 112. These features are formed in a cavity 116 formed in a substrate 118 through conventional etching techniques, and may be anchored to the underlying substrate 118 or cantilevered structures released from the substrate 118. The electrodes 108 and 110 are typically made of polysilicon or a material comprised of multi-films, such as silicon dioxide or aluminum, thereby creating individual parallel-plate capacitors between each adjacent pair of the interleaved electrodes 108, 110. In operation, when the accelerometer 100 is accelerated, the electrodes 108 move relative to the electrodes 110, thereby varying the distance, and hence the capacitance, between the electrodes 108, 110. The variable capacitance can be determined by peripheral circuitry interfacing with connectors 120, which are connected to the electrodes 110 via the support beams 112.
The sensitivity of such prior micromachined accelerometers 100 is dependent upon a number of factors, including the mass of the proof mass 102, spring 104 and capacitance between the electrodes 108, 110. Generally, the greater the mass of the proof mass 102 is, the better the inertial sensing device is because for a given acceleration, there will be a greater force. Thus, additional mass could be added by enlarging the proof mass 102, thus increasing the tendency of the proof mass 102 to remain motionless relative to the other components of the accelerometer 100. However, because typical prior micro-accelerometers require peripheral circuitry, there is a practical limit to the available size of the proof mass 102 given a particular die size. Besides, the greater the total capacitance between the electrodes 108, 110 is, the better the sensitivity of the accelerometer 100 is. The total capacitance between the electrodes 108, 110 is proportional to the total overlapping area between the electrodes 108, 110. An approach to increasing the total overlapping area between the electrodes 108, 110 is to increase the number of the electrodes 108, 110. However, the increase in the number of the electrodes 108, 110 typically needs an appropriate increase in the lateral sides for the proof mass 102 in order to accommodate more electrodes 108, 110. As described above, there is a practical limit to the available size of the proof mass 102 given a particular die size.
Accordingly, there exists a need to provide a micromachined sensor to solve the above-mentioned problems.